Shrink labels are an effective way of decorating the contours of a plastic bottle with graphics and text without having to apply ink directly to the plastic bottle and without adhesives. Adhesiveless labels enable transferring graphics to a bottle containing cold liquid, since the cold liquid would normally have condensed water on the surface of the bottle, which condensed water would interfere with most adhesives. Shrink labels are often made with heat shrinkable polymers such as, but not limited to, heat shrinkable polyvinyl chloride (PVC) and polyethylene terephthalate glycol (PETG) polymer films. There are three main methods used to heat polymer films to shrink them: radiant heating, forced dry air convection, and steam heating. To utilize any of these three methods, labeling equipment is normally comprised of a heating tunnel with a conveyor belt running through it.
For radiant heating, a hot surface is created inside a heating tunnel and the resulting infrared radiation strikes the polymer films to heat and, thereby, shrink them. The hot surface can be created using heat lamps or resistance wires. For many shelf-stable drinks, a mild temperature increase is acceptable, and radiant heating can work inexpensively.
For forced dry air convection, hot air is created by passing air over a heating element or a combustion process could be used—with or without a heat exchanger. The heated air is then blown over the surfaces of the bottle and shrink film and, as a result, the film shrinks snuggly over the bottle. Since dry air does not have as much heat capacity as steam, the spatial uniformity of the shrink is subject to non-uniformities of the heat transfer coefficient due to varying scales of turbulence and temperatures of swirling hot air. As a result, depending upon the speed and shape of the bottle traveling on the conveyor belt relative to hot air in the tunnel, there can be regions of the label that get distorted and wrinkled, which distortions and wrinkles can be practically impossible to eliminate by changing the air temperature, convection speed, and/or the conveyor belt speed. A reason for these distortions and wrinkles is that the bottle is positioned standing up with its axis of rotation perpendicular to the direction of motion of the conveyor. As a result, the flow of hot air over the bottle is non-uniform, i.e., a leading side of the bottle will have a relatively predictable flow pattern, whereas a trailing, or downstream, side of the bottle will separate into vortices. In addition, air flow near a narrower neck of the bottle sheds vortices due to complex geometries of bottle narrowing. Such vortices tend to detach from the bottle in an alternating and irregular rhythm, and this leads to uncontrolled hot air velocity fields. Further, such uncontrolled hot air velocity fields lead to unreproducible heat transfer coefficients for hot air that is in contact with a shrink label and results in distortions or local overheating.
Steam labeling has the advantage that, due to the inherent ability of steam to transfer a heat at lower temperatures due to heat released during a phase change from gas to liquid on the label surface, steam can shrink a label with fewer wrinkles and can do it more uniformly than dry air convection. A problem with steam labeling is that, if the steam is too wet, droplets of water can condense on the shrink film. This may cause water beads to interfere with heat reaching the film locally while drier areas of the film get more concentrated heat and, thereby, shrink faster. This can lead to non-uniformity and/or more heat going into the liquid inside the bottle. To prevent too much condensation, the steam often needs to be produced in a pressurized boiler and released into the atmosphere to produce a drier steam. The problem of a higher pressure boiler is the potential for boiler explosion and the associated costs of equipment and facilitization to manage the safety risks. In addition, problems with current steam shrink labeling tunnels are that they are large, expensive, and use a lot of steam. They often require a large boiler that in some cities require a separate room to be constructed for safety purposes. The tunnels are often complex machines, having computer controls and many hundreds, if not thousands, of parts—all of which increases the cost of purchase and maintainability of the machine—and, as a result, rendering the machine non-portable.